The present invention generally pertains to calibration of a bulk material analyzer and is particularly directed to the composition, use and manufacture of a calibration block for a bulk material analyzer.
Bulk material analyzers are used to measure the elemental content of bulk material. Such analyzers have been developed primarily to measure the quantitative content of materials, such as ash, in batches of coal. The parameters of interest may be determined from the measurement of the elemental content of the bulk materials.
In a typical prior art bulk material analyzer, the bulk material is transported through an activation region between a radiation source and a gamma ray detector, and the detector produces signals which are processed to provide a measurement of the elemental content of the bulk material. Typically the radiation source is a neutron source. When the bulk material absorbs neutrons, secondary emissions of gamma rays are produced from the bulk material. Different characteristic gamma ray energy spectra are produced from different elements in the bulk materials. Accordingly, by processing detected signals that are indicative of the gamma ray spectrum, a measurement is provided of the elemental content of the bulk material. This measurement process is known in the art as prompt gamma ray neutron activation analysis (PGNAA).
In the prior art, bulk material analyzers have been calibrated by using samples of materials of the type that are to be analyzed. Prior to calibration, the samples are subjected to laboratory analysis to determine their elemental content. Typically, the samples used in the chemical laboratory analysis are relatively small compared to the quantity of bulk material present in the activation region during operation of the bulk material analyzer. For example, whereas a coal analyzer typically has several hundred pounds of coal in its activation region during operation, a coal analysis laboratory uses only 50 grams (0.1 pounds) or less of finely crushed coal, from which it draws sub-samples for each of the necessary chemical analyses. Coal is a heterogeneous material. Hence, a major source of inaccuracy in any coal analysis is the collection of the sample of 0.1 pounds (50 grams) that is truly representative of several hundred pounds of bulk coal. According to the Coal Handbook, Meyers (Ed.), Dekker, New York, 1981 uncertainties in obtaining and preparing the sample are twenty times greater than a laboratory's analytical uncertainty.
Since the first development of large scale PGNAA coal analyzers a quick, easy and accurate method of calibration has been a problem. Over the past ten years published research from the Electric Power and Research Institute (EPRI) has identified the calibration problems and attempted to solve them by several different methods.
One such prior art method includes the step of uniformly and accurately "spiking" moving coal streams with elements and compounds. This method is described by R. F. Stewart et al.; Bureau of Mines Tech. Progress Report 74, 1984. This method requires a mechanical means to move the coal dynamically for even mixing. Only one element can be done at a time and once a coal has been spiked the coal must be discarded or re-used in its contaminated form. Segregation of the coal and spike material can give large inaccuracies. This method is not particularly easy, quick or accurate.
Another prior art method includes the step of mixing up a powdered plastic, or a carbohydrate sugar, and dry chemical compounds to simulate a coal matrix and thereby provide a standard material. This method is described by Duffey et al.; American Nuclear Society Transactions, Winter 1976; Vol. 24, p. 117. Using this method, different elemental coal types can be simulated. However, it is physically difficult to achieve controllable densities with powders. Contamination, especially from moisture, can easily occur. Hence, care is needed in using, handling and storing the standards. Even the most elaborate of blending methods cannot overcome the problems of segregation between lighter and heavier material components. Segregation gives inaccuracy. This method is quick but not accurate nor easy.
Still another prior art calibration method uses boxes of powdered coal that have been heavily sampled and then analyzed by many (3-5) laboratories and thus are assumed to be "standard materials." This method is described by Brown, Gozani & Spencer; Nuclear Assay of Coal, Vol. 10, EPRI Document RP, pp. 983-4, Nov. 1983. In this method, the box is analyzed simultaneously with the "standard material". The box thus represents a non-uniformly distributed contaminant to the "standard material". The density and freedom from segregation cannot be maintained upon transport and handling. Also, it is doubtful whether the "standard materials" will remain stable in density and moisture distribution over long periods of time when they are not hermetically sealed. This method is quick and easy, but its accuracy does not allow a calibration that will test a PGNAA bulk material analyzer to its limits.